Hard metals comprise a binder phase composed of cobalt and/or nickel and a hard material phase which can have, for example, WC, TiC, TaC, NbC, VC and/or Cr3C2.
Such hard metals are differentiatable from cermets by having a greater proportion of a TiCN phase and include other carbides and/or nitrides. Binder metals also include the elements of the iron group, usually Co and/or Ni.
Ceramics, especially for machining purposes are comprised usually of Al2O3 and/or ZrO2. Depending upon the machining operation and the workpiece to be machined, the wear-resistance and useful life can be increased by single-layer or multilayer coatings of the above-mentioned compositions. The coating can be applied by means of a physical vapor deposition process (PVD) or a chemical vapor deposition process (CVD) whereby the CVD process or—in a further development—a so-called plasma supported CVD process (PCVD) has the advantage of uniform coverage which avoids the shading effects arising with the PVD process.
The microstructure, the residual stresses or intrinsic stresses and the adhesion stability of single-layer or multilayer coatings is strongly dependent on the coating process used and the coating parameters employed. In the techniques common in the past, CVD deposited coatings as a rule showed tensile stresses while those coatings deposited by PVD processes were characterized by compressive stresses. To improve the resistance to breaking it has-been proposed, for example, in WO 92/05296 to combine a CVD layer or a plurality of CVD layers with one or more layers deposited by PVD, whereby as the material for the internal layers deposited by CVD, nitrides of titanium, hafnium and/or zirconium were used and for the layers deposited by means of PVD, nitrides and carbonitrides of the mentioned metals were proposed. Disadvantageously, however, such a coating requires different apparatuses and both involve considerable work and is costly.
In DE 197 19 195 A1 it has been proposed to deposit a multilayer coating by an uninterrupted CVD process at temperatures between 900° C. and 1100° C. by alteration of the gas composition from time to time. The outer coating (cover layer) is comprised of a single phase or multiphase layer of carbides, nitrides or carbonitrides on a Zr or Hf basis which is deposited by means of CVD and has intrinsic compressive strength and an underlying layer or layers also applied by CVD and exclusively with intrinsic tensile stress whereby at least one underlying layer or the single underlying layer is comprised of TiN, TiC and/or Ti (C,N). The compressive stresses measured in the outermost layer or the outer layer lies between −500 and −2500 MPa (compressive stresses are defined with negative values by comparison with tensile stresses for which positive values are used).
According to the state of the art it is further known to subject coated substrate bodies after coating to a surface treatment. Current mechanical treatment methods are brushing and blast treatment in which the blast agents used are ball-shaped particles with a grain size from 300 μm to 600 μm directed against the surface by means of compressed air at a pressure of 2×105 to 4×105 Pa. Such a surface treatment increases the compressive residual stress of the outermost layer by slight hardening. In this manner detrimental crack formation and spreading, corrosion and peeling or chipping reactions are encountered.